1. Field of the Invention
The present invention relates generally to corneal topographers and more particularly, to a unique placido plate arrangement.
2. Description of the Related Art
The use of reflected placido rings or patterns to analyze a patient's eye are well-known. Various methods of positioning and illuminating a plurality of placido patterns in front of a patient's cornea have been described in the prior art. Such methods include photokeratoscopes wherein rings are disposed on a plain surface, which is held in front of the patient's cornea and illuminated by a light source to one side of the cornea. Another method is reflecting rings formed on the interior surface of a sphere with the light source outside the sphere, or reflecting rings formed on an inner surface of a cylinder with a light source disposed outside of the cylinder. Yet another method is reflecting rings disposed on the interior surface of a cylindrical cage with the light source disposed within the cage itself.
Illuminating the placido patterns causes reflections of the patterns to appear on the cornea, and these reflections are then depicted, optically or photographically, and then analyzed by an eye specialist. Typically reflected placido rings are circular, and deviations of the cornea from a sphere cause bumps or indentations indicative of irregularities in the cornea, which irregularities will be present in the reflection of the placido patterns. As described in U.S. Pat. No. 5,864,383 to Turner, et al. issued Jan. 26, 1999, the contents of which are incorporated by this reference, sets forth that it is sometimes desirable to change the image or pattern illuminated by an apparatus. For example, a placido ring pattern may be preferred to a checker board pattern or still more preferably, a spider-web pattern as described in co-pending U.S. patent application Ser. No. 10/261,539 may be preferred. As set forth in the patent, changing a pattern on a traditional placido plate was difficult, i.e., on a spherical dish shape placido screen. As taught by Turner, et al., it was an improvement in the art to have an easy to manufacture, non-planer screen, which could be readily associated and disassociated with chosen patterns.
While the invention disclosed by Turner, et al. has proved very successful, it would be desirable to have a placido screen that would provide more surface area coverage of a patient's cornea, while still providing an easily manufactured screen that will easily accommodate a change in placido patterns.